1. Field of the Invention
This invention relates to heat-reflecting films. More particularly, it relates to composite films comprising a series of dielectric and metal layers so as to create an infrared reflecting interference filter and to the use of such films in window glazing materials.
2. Description of the Prior Art
In the 1890s, Fabry and Perot developed an interferometer consisting of a pair of parallel-sided, half-silvered mirrors separated by a nonabsorbing layer. This device had the property of preferentially passing energy of certain wavelengths and reflecting energy of other wavelengths. An embodiment of this principle known as the Fabry-Perot sandwich consists of two more or less transparent metal layers separated by a dielectric spacer layer. (See, for example, Knittl, Zdenek, OPTICS OF THIN FILMS, John Wiley & Sons,Ltd., London, 1976, at page 284.) Other filter products known as "induced transmission filters" have been constructed of metal-dielectric sandwiches for use in window glazing structures. One such structure is described in U.S. Pat. No. 4,337,990 of Fan (July 6, 1982) as consisting of a transparent substrate, overlayered with a phase matching layer, a single metallic silver layer and an outer antireflection layer, with the three overlayers constituting a transparent heat reflector. While generally effective, products of this general structure suffer from the disadvantage that to achieve high levels of heat reflection they must have relatively thick metal layers such as 15 to 25 nm in thickness which tend to have low transmittances of visible radiation, as well.
Another system which used a Fabry-Perot approach to achieve heat reflection while transmitting visible radiation is shown in U.S. Pat. No. 3,682,528 of Apfel and Gelber (Aug. 8, 1972). In this system, thinner layers of metal are employed but it is taught that to obtain such layers of an optically suitable metal, in particular silver, it is necessary to first lay down a thin "nucleation" precoat layer of nickel by vacuum deposition and then apply the silver to it, again by vacuum deposition methods. It is further taught that the deposited silver must then receive a thin postcoat layer of vapor-deposited nickel if another layer is to be applied over it. These extra coatings with nickel are time consuming and economically unattractive. This patent also discloses a filter having two silver layers but shows that each silver layer must be accompanied by one or two nickel layers and suggests only durability advantages to this more involved structure. The substrate upon which this multilayer heat-reflecting film was constructed most commonly was glass.
It is also recognized that a wavelength selective filter can be achieved with a stack of alternating high/low index of refraction dielectrics. This would work but would require a large number of layers and be prohibitively expensive. This would not have the capability of low emissivity either.
In U.S. patent application Ser. No. 880,797, from which this application descends, we described the application of the Fabry-perot approach to glazing products. While as a general class these materials perform the task of heat rejection with admirable efficiency, in some settings they present a strong color cast to their reflection when viewed from the incident light direction. This strong color is often objected to by some consumers. It is an object of this invention to correct this failing in heat reflective glazings.
Another desired property of reflective glazing is that the appearance does not change as a function of viewing angle This absence of angle sensitivity has not been easily achievable with prior materials used in Fabry-Perot configurations.
In studying glazing materials employing these heat-reflecting filters we have discovered a number of configurations for plate glass and anti-lacerative glass which optimize the effectiveness of the filter systems and/or simplify their fabrication. These glass configurations find application in automotive and architectural settings. In some automotive settings, there is a desire to reflect as much heat as possible, but this must be done within the confines of various regulations setting light transmission limits and the like. For example, in the United States, automotive windshields must have a transmission of visible light of at least 70% at normal incidence. The present invention can serve these needs.